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Computer Aided Interferometry


We will be offering selected optics from stock with serialized print outs (just like the sample of the 10" f/6 mirror below). Computer-aided interferometry is the most accurate method used today for optical analysis... PERIOD. Accept NO SUBSTITUTION!!. Please contact us if you are interested in this specialized report. Tell us what size optic you are looking for and if it's not in stock, we'll let you know when it will be. We will forward to you, via e-mail, the test report so you may inspect it prior to purchase. The price will be higher than those listed on our web page.

We will also manufacture any optic we sell with the computer-aided data analysis, pricing will vary depending on the size of the optic. We will test any optic up to 24" and provide detailed documentation of our results, inquire for pricing/lead time.

  Interferometry is incredibly accurate and precise. When combined with computer - aided analysis software, you get a true representation of the optic under test. The software can "read" several hundred or even several thousand points on a optic or system, where as someone reading an optic with a Foucault tester is checking only 4 to 6 zones (not really points). The software can also combine multiple interferograms (3 are used in this example) of the same optic and average several together. Typically, 2 to 8 interferograms are used for the data average depending on the size of optic under test.

All our optics are hand figured against an over size optical flat mirror, this set up is called a double-pass, auto-collimation test. We incorporate the use of a Foucault and Ronchi test to reduce visible error to the very minimal amount possible, once we reach this point are we done yet?, maybe and maybe not.........

This is where the computer aided interferometry comes into play. Optics that exhibit a good test result under the double-pass/auto-collimation may in fact have errors greater than one would generally accept. Straight lines with a Ronchi screen may indicate a good (qualitative) optic, but just how good (quantitative)? When an interferometer is placed into the test the "bands" or fringes will usually exhibit a lot more bumps and dips, curves and bends than one would see with the other testers.

Why is this? Well, the interferometer is just that much more sensitive. It measures the wave front of the optic. It shows far more detail than other types of testers, this is why it is the only preferred method of testing accepted by serious purchasers of fine optics.

Lets take a closer look at the example; it exhibited straight "bands" with a Ronchi test under a double-pass/auto-collimation test (see photo) and a smooth surface with a Foucault test. So lets see how the interferometer rates the same mirror.

What?... the lines aren’t straight!

The optic tested 1/5 wave at the wave front, an excellent optic. Despite the very respectable report you can see that the "bands" or fringes are not straight. It is very difficult to get straight fringes with interferometry, it is THAT SENSITIVE. Please do not confuse Ronchi-grams (that others may supply) with interferograms, they are not the same thing. I have included one Ronchigram (bottom right) so you can compare it with the interferograms.

The three interferograms (left) were combined and averaged together to achieve the data results. All images are of the same mirror.

When reading data there are really three items to look at, they are;
1) RMS - this should be 0.075 or lower (smaller).
2) Strehl ratio - this should be 0.80 or higher.
3) PVWF - this should typically be about 0.250 or better, but remember this is the difference between the highest and lowest point on a optic. An optic can have a greater PVWF rating and still be a great mirror if the RMS and Strehl ratios good.

It is best to judge an optic in the above order, with RMS and the Strehl ratio being the most important data to be concerned with as they represent the entire surface. It is very possible to have a mirror with a PVWF greater than 0.250 (1/4 wave) and it still will be a fine optic, passing the star test and giving detailed views of the planets. Remember, the RMS and Strehl numbers will give the best over-all view of the quality of the optic or system under test.

Do not buy into the hype of the overly inflated PV claim, most do not have interferometry to verify their own work and if they did they would not advertise the numbers that they do. We recently tested a mirror from another maker that was claimed to be 1/47 PVWF and had "documentation" to prove it. It tested out very well at 1/5 PVWF, a very fine mirror, but not 1/47 wave. Testing optics with a Foucault tester at radius of curvature and comparing 4 to 6 zones will not give a "truly" accurate depiction of the optic, plus the human element of actually reading the zones becomes a factor in achieving unbiased test data.





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